This invention relates in general to prosthetic devices such as implants and artificial joints and more particularly to such devices which are associated with replacement of the knee joint.
The knee joint basically consists of the bone interface of the distal end of the femur and the proximal end of the tibia. Appearing to cover or at least partially protect this interface is the patella which is a sesamoid bone within the tendon of the long muscle (quadriceps) on the front of the thigh. This tendon inserts into the tibial tuberosity and the posterior surface of the patella is smooth and glides over the femur.
The femur is configured with two knob like processes (the medial condyle and the lateral condyle) which are substantially smooth and articulate with the medial condyle and the lateral condyle of the tibia, respectively. The condyles of the tibia are substantially smooth and slightly cupped thereby providing a slight receptacle for receipt of the femur condyles.
When the knee joint is injured whether as a result of an accident or illness, the natural bones may become damaged to the extent that they are unable to function (articulate) properly. If the bones are affected beyond the level or degree where natural healing and new growth will remedy the damage, then a prosthetic replacement of the damaged portion is called for in order to restore normal use to the joint. Typically the entire joint is replaced by means of a surgical procedure which involves removal of the ends of the corresponding damaged bones and replacement of these ends with prosthetic implants. A typical such implant would be for the hip joint wherein a metal implant could be anchored in the intramedullary canal of the femur and would provide a generally spherical protuberance extending outwardly therefrom. The mating prosthetic portion would be a polyethylene socket member suitably anchored into the acetabulum. While prosthetic devices of this type, normally including a physiologically inert metal member and an engaging high density polyethylene member, are well known in the art, these types of devices are of a fixed and unchanging nature once they are inserted (implanted) into the patient and anchored there, whether by pinning or by acrylic bone cement or both.
Due in part to the fact that the size, shape and anatomy of virtually every patient is different, great care must be taken by the orthopedic surgeon in order to select properly sized and shaped prosthetic members for implanting. In order to achieve a suitable fit and size compatability an extensive number of variety of each type of prosthetic implant must be available to the orthopedic surgeon from which he may choose. As a result the cost of inventory as well as the logistics of ordering and storing a wide variety of prosthetic implants is cumbersome. Therefore, it would be an improvement to this present situation if prosthetic devices could be structured with removable portions such that there would be a reduction in inventory without a corresponding compromise as to the number and variety of different prosthetic combination which can be created. In order to provide such an improvement it is necessary that the prosthetic portions which are to be mixed and matched into a variety of combinations be suitably secured together so that the prosthetic member, which they in combination create, is not weaker nor more likely to fail than would be a similar prosthetic member constructed as a single integral piece.
A further concern involves the procedure when a prosthetic device becomes worn or damaged and a replacement must be made. While this is possible, it oftentimes involves elaborate surgery depending upon the particular portion of anatomy involved and the extent or nature of the damage to the prosthetic device. Furthermore, certain portions of the anatomy such as knee joints may be more susceptible to wear due to the surface area of articulation, the nature of the anatomy and the typical loads and forces which are encountered by this particular joint. Repair and/or replacement may also be desired when interfacing portions of the anatomy change and the contacting portion of the prosthesis needs to be revised as to its shape or size.
By structuring a prosthetic implant such that the portion most likely to wear or desired to be changed is quickly and easily replaceable from the remaining portion of the prosthetic implant, significant amounts of surgical time can be saved and the prosthesis can be more closely tailored to the patient's needs. Equally important is the fact that the portion of the prosthesis which is anchored into the patient, such as a tapered shaft inserted into the intramedullary canal of the tibia, does not have to be surgically removed in order to make a replacement of a worn or damaged tibia articulation portion. Such a replaceable concept, in order to be effective, must securely hold the anchored portion and the replaceable portion together so as to act as an integral member regardless of the nature or complexity of the forces and loads acting thereon. With a design which achieves the requisite strength and durability, it is then possible to mix and match the replaceable portion with various anchored portions and vice versa such that, for example, an inventory of three replaceable portions and three anchored portions for a tibia implant would be able to provide nine different combinations to the orthopedic surgeon rather than having to inventory and stock nine separate complete tibia implant members.
The prosthetic device disclosed herein provides means and apparatus for such a replaceable prosthetic concept and achieves the various objects discussed as well as others as will be apparent from the disclosure which follows.